Solid-state image sensors, also known as imagers, were developed in the late 1960s and early 1970s primarily for television image acquisition, transmission, and display. An imager absorbs incident radiation of a particular wavelength (such as optical photons, x-rays, or the like) and generates an electrical signal corresponding to the absorbed radiation. There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCDs), photodiode arrays, charge injection devices (CIDs), hybrid focal plan arrays, and CMOS imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems and data compression systems for high-definition television.
These imagers typically consist of an array of pixel cells containing photosensors, where each pixel produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image. The photosensors are typically phototransistors, photoconductors or photodiodes, where the conductivity of the photosensor or the charge stored in a diffusion corresponds to the intensity of light impinging on the photosensor. The magnitude of the signal produced by each pixel, therefore, is proportional to the amount of light impinging on the photosensor.
It is known in the art to use a microlens array with an imager array, wherein the microlens array comprises a convex microlens for each pixel. The microlenses refract incident radiation from the circuitry region of the pixel to the photosensor region, thereby increasing the amount of light reaching the photosensor and thereby increasing the fill factor of the pixels. Other uses of microlens arrays include intensifying illuminating light on the pixels of a nonluminescent display device such as a liquid crystal display device to increase the brightness of the display, forming an image to be printed in a liquid crystal or light emitting diode printer, and as focusing means for coupling a luminescent device or a receptive device to an optical fiber.
Despite the use of microlens arrays, a large amount of light incident on an imager is not directed onto the photosensor due to the geometry of the microlens array. In particular, light incident on the space between individual lenses (the lens-lens space), and on the edges of the pixel beyond the edges of an individual lens remains uncaptured by the microlens, and never impacts the photosensor. Additionally, the typical practice of forming the microlens array on a separate substrate from the pixel array leads to problems of lens-pixel alignment that results in additional lost light.
There is needed, therefore, a microlens array having an improved fill factor formed on the same substrate as a pixel array. A simple method of fabricating a microlens array having an improved fill factor is also needed.